A number of problems are encountered when connecting and disconnecting an electrical generator to a circuit carrying an existing alternating current such as the main electricity supply.
An electrical generator can be started in isolation until it reaches suitable power quality and the appropriate frequency and phase, and then connected to the main electricity supply. However, the provision of a control device to monitor the power quality of a generator and then connect the generator to the main electricity supply when the appropriate power quality has been achieved is expensive, increases the complexity of the device and is liable to error.
When disconnecting the electrical generator from the main electricity supply, arcing is produced between the contacts of the switch used for disconnection. This arcing wears out the contacts of the switch impairing performance and necessitating regular replacement which is inconvenient and expensive. Arcing also disturbs the quality of the main electricity signal which is prohibited. Arcing can also cause damage to other components and the starting of fires and its prevention by for example the provision of an inert gas atmosphere is expensive.